Ask A Scientist

What is sand made of?

Answer from Douglas W. Green

Adjunct lecturer, Binghamton University School of Education

Research areas: Leadership and Instructional data analysisFamily: Wife Denise and daughter Lena age 24 who is an animator in New York City. Interests/hobbies: Taking care of my wife who has ALS (Lou Gehrig’s Disease), reading, and golf

Have you ever strolled along a beach or played in a sandbox and wondered just what sand was? Sand is made of small pieces of some kind of rock or mineral. Minerals have a specific chemical composition while rocks are mixtures of minerals. You can think of a single grain of sand as a small rock. Various scientists and engineers have their own definitions for how big a piece of rock can be and still be considered as sand. Roughly, any grain that is between 0.1 mm and 2.0 mm in diameter can be considered sand. A ruler with metric measurements can give you some idea of how big this is.
Sand particles are small enough so they can be easily moved by wind or water current. They are also big enough so they do not stay suspended for long if the wind or water current slows down. Sand particles are also big enough so you can feel them if you rub them between your fingers but they are smaller than the head of a match. Smaller particles would be considered to be silt or dust and larger particles would be called gravel. You can feel the difference if you rub some table salt and flour between your fingers. Table salt has sand-sized particles but flour does not.
Since there are so many types of rocks on the earth there are also many types of sand. A common component of many types of sand is called silica, which has a chemical formula of SiO2. This formula tells us that there is one atom of the element silicon for every two atoms of oxygen. Usually these atoms are aligned in a regular three-dimensional pattern known as a crystal. This substance is also known as the mineral quartz. Even quartz-based sand usually contains a mix of other minerals and some sand contains no quartz. Since most minerals and rocks can be crushed into sand-sized particles there are almost limitless combinations. They vary from pure white sand made of limestone to pure black sand made from lava. Sand can also be compressed to make a rock called sandstone.
Sand is a major construction material. The concrete used to make many buildings and road surfaces contains grains of sand held together by cement, which is a kind of glue made from crushed limestone. A lot of the sand we use for building is manufactured by crushing rocks. This allows builders to control the mineral composition and grain size. Even the sand used for beach volleyball in the Olympics is manufactured. Sand with a high silica (quartz) content is also the main constituent in many types of glass.
Sand particles that have formed recently by natural or man-made processes feature grains that have sharp edges. Sand that has been exposed to wind or water for long periods of time will have rounded grains. You can take a look at any samples of sand you can find under a magnifying glass or low-powered microscope to see if the grains are rounded or have sharp edges. This will also allow you to see if the sand is relatively pure or composed of a mixture of different colored minerals.